The present invention generally relates to semiconductor devices and methods of producing the same, and to semiconductor device units and methods of producing the same. More particularly, the present invention relates to a semiconductor device which encapsulates a semiconductor element by a resin and to a method of producing such a semiconductor device, and to a semiconductor device unit and a method of producing the semiconductor device unit.
Recently, there are demands to further reduce the size of electronic equipments such as personal computers, and as a result, there are increased demands to realize smaller semiconductor devices which are used in such electronic equipments.
In semiconductor devices, a substrate which holds a semiconductor element or the volume of an encapsulating resin is large compared to the size of the semiconductor element. For this reason, in order to reduce the size of the semiconductor device, it is necessary to appropriately reduce the size of the substrate or the encapsulating resin.
On the other hand, when the size of the semiconductor device is reduced, the heat radiation characteristic of the semiconductor device deteriorates. As a result, it is necessary to provide an efficient heat radiation means.
Therefore, there are demands to realize a semiconductor device which has a reduced size but can be formed with ease and has a satisfactory heat radiation characteristic.
A description will be given of an example of a conventional semiconductor device and a method of producing the same, by referring to FIGS. 1A through 1C. FIGS. 1A through 1C respectively show cross sections of the conventional semiconductor device at various stages of the production process.
As shown in FIG. 1C, a conventional semiconductor device 60 includes a semiconductor element 1, a mold resin 3, a lead frame 46 and the like. The semiconductor element 1 is die-bonded on a die-pad portion 39 which is formed on the lead frame 46. In addition, wires 10 are provided between electrode pad portions 40 on the top surface of the semiconductor element 1 and inner lead portions of the lead frame 46. The electrode pad portions 40 are shown in FIG. 1A. The mold resin 3 encapsulates the semiconductor element 1, the wires 10 and the inner lead portions of the lead frame 46, so as to protect the semiconductor element 1 and the wires 10.
The mold resin 3 is formed as shown in FIG. 1B. In FIG. 1B, an upper mold 20a and a lower mold 20b form a transfer mold. When the lead frame 46 mounted with the semiconductor element 1 and having the wires 10 bonded thereto as shown in FIG. 1A is loaded between the upper and lower molds 20a and 20b.
A plunger 22 arranged on the upper mold 20a heats and presses a resin tablet (mold resin 3) which is indicated by a dot-pattern, so that the melted resin tablet flows into a cavity part formed between the upper and lower molds 20a and 20b via a cull part 43, a runner part 44 and a gate part 45. The cavity formed between the upper and lower molds 20a and 20b has a shape corresponding to the outer shape of the semiconductor device 60. Hence, by filling the rein into this cavity part, the mold resin 3 is formed into a package having a predetermined shape.
According to the conventional resin filling method, the mold resin 3 remains at the cull part 43, the runner part 44 and the gate part 45, integrally to the package part of the semiconductor device 60. Accordingly, when the semiconductor device 60 is removed from the upper and lower molds 20a and 20b, the remaining resin is broken at the gate part 45 and the broken off resin is thrown away.
On the other hand, as is well known, the semiconductor element 1 generates heat when driven. In the case of the semiconductor device 60 which is produced in the above described manner, the heat generated from the semiconductor element 1 thermally conducts via the mold resin 3, and the heat radiation takes place particularly at the back surface of the package where the thickness of the mold resin 3 is thin. The back surface of the package is the bottom surface of the mold resin 3 in FIG. 1C.
As described above, the main function of the mold resin 3 is to protect the semiconductor element 1 and the wires 10. However, the mold resin 3 must also have a satisfactory adhesive strength. In other words, if the adhesive strength of the mold resin 3 with respect to the semiconductor element 1 and the wires 10 provided within the mold resin 3 is weak, the semiconductor element 1 and the wires 10 may move within the package. If the semiconductor element 1 and the wires 10 move within the package, it becomes impossible to positively protect the semiconductor element 1 and the wires 10, and the reliability of the semiconductor device 60 will deteriorate.
But when carrying out the transfer molding, it is essential to separate the upper and lower molds 20a and 20b from the molded package, and the upper and lower molds 20a and 20b can be separated more satisfactorily if the above adhesive strength is not too strong. For this reason, a mold release agent which facilitates the separation of the upper and lower molds 20a and 20b is conventionally added to the mold resin 3. The kind and quantity of this mold release agent that is to be added are selected to optimize the balance between the reliability of the semiconductor device 60 and the mold release (or parting) characteristic of the upper and lower molds 20a and 20b with respect to the molded package. More particularly, out of the cull part 43, the runner part 44 and the gate part 45, the mold release characteristic is poorest at the gate part 45 where the flow passage of the mold resin 3 is the narrowest. Hence, the kind and quantity of the mold release agent to be added are generally selected with reference to the mold release characteristic at the gate part 45.
However, as the size of the semiconductor device 60 is further reduced and the package becomes smaller, the cull part 43, the runner part 44 and the gate part 45 all become smaller, thereby deteriorating the mold release characteristic. For this reason, it is necessary to increase the amount of the mold release agent that facilitates the mold release as the package becomes smaller, but there was a problem in that the reliability of the semiconductor device 60 deteriorates if the amount of the mold release agent is increased for the reasons described above.
On the other hand, as for the heat radiation characteristic, the conventional semiconductor device 60 is designed to radiate heat via the back surface of the package. For this reason, there was a problem in that the heat generated from the semiconductor element 1 could not be released efficiently. In addition, there have been proposals to provide a plurality of radiator fins independently on the package, but the provision of the radiator fins increases the overall size of the semiconductor device 60, and there was a problem in that the provision of the independent radiator fins cannot realize the size reduction of the semiconductor device 60.